2.1 Manure management systems
2.2 Emissions before manure application
2.3 Effects on soil quality
2.4 Effects on ground water and surface water quality
2.5 Effects on air quality
2.6 Effects on crops
2.7 Parameters influencing effects of manure on the environment
Manure management systems are highly diverse, among which the following can be distinguished:
-Grazing. Substantial losses through leaching may occur due to the uneven distribution of faeces and urine (urine patches may have a N load equal to 200-550 kg/ha; Van der Meer and Meeuwissen, 1989; Romney et al., 1994). Volatilization of N may also be considerable (10-25%, see 3.3.5), but less important since part is deposited on nearby areas, though some of it on non-agricultural land.
-Kraals. These enclosures are often used as in-situ fertilization of arable land by moving the kraal regularly. Soil fertility of a larger area, used for grazing, is partially concentrated on the arable land, thus enabling crop production in resource-poor situations. Losses through leaching will be slightly higher than during grazing as equivalent N and K fertilization rates are increased.
-Dry lot storage. If urine is not collected and bedding is sparsely used, losses of N and K in particular will be high as most urine is lost. Depending on the storage facilities and storage time of the faeces part of the nutrients in faeces will also be lost through leaching and surface runoff, in the case of a precipitation surplus and uncovered manure heaps. Urine collection will minimize K losses but N losses will often remain high as volatilization will increase, though this is dependent on climatic conditions, storage time and storage method. Using bedding, with sufficient absorption capacity to capture urine, might reduce N losses with ca. 15% of the mineral N (see 3.2.2).
-Slurry storage. This system of manure storage, where faeces and urine are stored together, is the main system in intensive livestock systems in OECD countries, except for broilers. Volatilization losses are dependent on the level of ventilation, depth of storage tanks and storage time, but often range between 5 and 35% of the total N excreted (see 3.3.2).
-Lagoons. Lagoon systems are quite common at large livestock farms in Eastern European countries and, to a lesser extent, in Asia, while their importance is growing in the USA. Liquid manure, either before on after separating part of the solids, is treated in anaerobic lagoons. Organic material is decomposed, thereby mineralizing part of the nutrients. The liquid phase is either discharged into surface water or used for irrigation. The main problems are related to the discharge into surface water (see 3.4.1), leaching through the lagoon bottom, and odour. High NH3 emission will occur as a major part of the N in mineral form, while also high CH4 and N2O emissions are also common.
-Plastering material for house construction. This is particularly important in Africa, however the amount of manure involved on a global scale is considered to be too insignificant to be discussed here. In this system all nutrients are lost for agriculture.
-Fuel. In many developing countries, and particularly in India, manure is an appreciable fuel. If burnt directly, most of the C, and all the N and S will be lost; other nutrients may be recycled to arable land via the application of the ash. The production of biogas from manure is another method to valorize the energetic value of manure. The high water content of the slurry makes it more difficult to handle, and N losses via volatilization may be high, because most N in slurry is in mineral form. Though strongly promoted (e.g. in China) and applied to some extent in Asia, its present application is still limited mainly due to high investment costs (both for the digester and adjoining equipment) and technical problems (Henglian, 1994).
-Feed. Manure could be recycled by feeding it to animals, both livestock and fish (Müller, 1980), but this practice is limited. In addition to widespread reluctance to use manure as feed, probably originating from fear of health hazards, this can be explained by the low nutritive value of most types of manure, except for poultry manure as ruminant feed which is of a reasonable quality (ca. 55-60% TDN, 20-30% CP). Consequently, in intensive production systems where collectable manure is abundant, more economic feed is available, while in production systems where the use of low quality feeds is common, high collection costs and/or opportunity costs (manure as fertilizer or fuel) are prohibiting the use of manure as feed. Therefore, no more attention will be paid to this subject in this report. A recent overview on this subject, however, is given by Sanchez (1994).
Stables and manure storage are major sources of ammonia (NH3) emission. About half of the N in manure (i.e. liquid manure or slurry) is NH3-N in solution. Because of the high vapour pressure of the NH3, it will readily volatilize upon exposure of the manure to the air. The greater the exposure, i.e. a larger specific area in contact with the air, the more NH3 volatilization.
If manure is stored in direct contact with the soil, the liquid can seep into the soil and into the ground water. The nutrients N, P, K, organic and other compounds can leach into the ground water and the manure storage thus acts as a major source of ground water pollution.
Surface runoff from farmyards, kraals, bomas and manure storage can be an important source of pollution.
Manure application to agricultural land involves the addition of all the components of the manure to the soil. An appropriate balance should be maintained between agronomic requirements and negative environmental impacts. Negative impacts, that could be defined as soil pollution, have to do with the addition of heavy metals, organo-chlorines and too many salts. Also, weed seeds could be spread through manuring the land. On the other hand, manuring almost always has a positive influence on the build up of soil organic matter and thus improves the "intrinsic" fertility of the soil, as well as the soil structure.
After application of manure, decomposition by microorganisms of the organic material will start into carbon dioxide (CO2), water (H2O) and minerals of plant nutrients such as N, P, S and metals. The transformation of organically bound elements into plant available nutrients during microbiological decomposition is called mineralization. Organic matter that remains one year after application is assumed to be part of the soil organic matter and will decompose gradually over the years, releasing plant nutrients in a way that resembles a slow release fertilizer. A more fertile soil, consequently, has less need for mineral fertilizers. The fertilizer industry uses non-renewable resources such as fossil fuel, phosphorus and potassium deposits, and is a source of emissions. Manuring has, in an indirect way, a positive effect on the environment (see Annex V for fossil energy requirements of inorganic fertilizer production). A small fraction of the added organic material is transformed into organic matter that is resistant to microbiological breakdown, the so-called humus or stable organic matter. Humus contributes to soil fertility by retaining plant nutrients through adsorption. It also acts as binding material in the soil, improving soil structure and is responsible for making clay soil less susceptible to compaction caused by heavy traffic, or a silty soil less susceptible to erosion. In addition, humus increases the water holding capacity and the cation exchange capacity (CEC) of any type of soil.
The heavy metals Cu and Zn have been mentioned as major contaminants from the heavy application of pig slurry (e.g. in part of The Netherlands). Repeated application of large doses of pig slurry to the same plot may lead to Cu and Zn levels in the soil that are toxic, for instance, to soil fauna and sheep. Since the 1978 EC legislation Cu additions to pig feed have been reduced increasingly, to a level of 35 mg per kg for growing and finishing pig feeds. At current levels, Cu and Zn are considered not problematic if P fertilization is in balance with the crop requirements (Fleming and Mordenti, 1993; see also 3.5.1).
There is a danger of incomplete degradation of organo-chlorines by microorganisms. Through manuring, they could be taken up by crops and pose a threat to humans by accumulating somewhere in the food chain (L'Hermite et al., 1993). Many countries have replaced organo-chlorines with organo-phosphates, but residues from insecticides still continue to be the main source of organo-chlorines in feed. Organo-chlorines originating from substances used against ecto-parasites can also be found.
The passage of weed seeds through the digestive tract of animals reduces their germination capability. Some weed species, however, survive. In a stack of farmyard manure, the temperature rises above 55 °C because of the microbiological decomposition of the organic matter and kills weed seeds within three weeks. The germination capability of weed seeds stored in slurry is destroyed only after a period of five months (L'Hermite et al., 1993).
Manure contains much dissolved potassium chloride (KCl) and sodium chloride (NaCl). Repeated application of large amounts of manure in arid or semi-arid climates may easily lead to salinisation of the soil, making it unsuitable for many crops (Sequi and Voorburg, 1993).
The main dangers of the application of manure are runoff of manure or manure components into surface water and leaching of nitrate (NO3) and P into the ground water. Mineral N in manure is largely present as NH3. If, upon application of the manure, it does not volatilize, it will be quickly nitrified, i.e. transformed through microbial action into NO3. Also, N mineralized from the organic fraction of the manure, will readily be nitrified. As NO3 is an anion that is not adsorbed by clay minerals or soil organic matter, it is easily leached in case of a precipitation surplus. This holds good for NO3-N from manure, and for that originating from mineral fertilizers or from decomposed soil organic matter. If ground water concentrations of NO3 become too high, it is unsuitable for drinking water. Under certain conditions, ground water can flow into surface water. In brackish and salt water in particular high NO3 concentrations in surface water will lead to eutrophication. Under certain conditions this may lead to excessive growth of algae, causing oxygen shortage and consequently the death of fish.
Phosphorus is not nearly as mobile in the soil as NO3 and therefore much less susceptible to leaching. Nevertheless, leaching of P can occur under certain conditions. Many sandy soils in The Netherlands have become "saturated" with phosphate (P2O5) after many years of heavy doses of manure. When saturated, the soil loses part of its capacity for P retention and leaching occurs. If P flows into the ground water and subsequently into surface water, the same problems as described above for NO3 will occur. Note, P causes eutrophication in freshwater bodies in particular.
Two processes involving N and one involving carbon (C) from manuring have an important effect on air quality. First, surface application of manure, particularly liquid manure, may cause substantial losses of NH3 by volatilization. In the Netherlands, for instance NH3 volatilization from manure is a major contributor to acid deposition. Unlike SO2, a contributor to acid deposition from cars and industry, most of the emitted NH3 is deposited near the emission source. Forests near regions with a high livestock density, are in a poor condition because of soil acidification caused by NH3 deposition originating from the livestock industry. Acidification may lead to mobilization of aluminium (Al) ions, which are very toxic to fish, disturbs the nutrient uptake of plants and trees, and enhances sensitivity to stress factors like drought and fungi. Besides the acidifying effect, NH3 deposition accounts for a considerable N load to the environment, causing eutrophication problems and N enrichment of the soils in nature reserves. The last mentioned can cause undesirable changes in species composition (important for biodiversity).
Second, denitrification of NO3 by microorganisms is possible under anaerobic conditions when N2 is formed, but giving off a by-product N2O, a gas that affects the ozone layer. Although quantitative data are scarce, animal excreta and arable land may be important sources of N2O globally.
A third important air pollutant is methane (CH4), formed upon decomposition of manure under anaerobic conditions. If stored manure is disturbed, CH4 will escape into the atmosphere and eventually, like N2O, affect the ozone layer.
In addition to CH4 formation in manure storage, the use of manure in flooded rice production (anaerobic conditions) and CH4 formation in the rumen of ruminants are important sources of CH4 emission. Methane and its consequences are discussed in detail in one of the other reports.
Odour has a negative effect on the air quality, affecting animals and people in closed stables as well as people near farms producing or applying manure. Especially in combination with dust odour may cause serious health problems. In the USA it is estimated that 70% of the workers in closed animal stables suffer from respiratory illness (Scialabba, 1994), this is one of the main causes of labour disability of pig farmers in the Netherlands. Hydrogen sulphide and ammonia are the main contributors to odour problems, though a dozen other compounds, like fatty or organic acids, phenols, etc. may also play a role. Most of the odour comes from the anaerobic decomposition of manure. This aspect of manure is not further discussed in this report.
Manure is applied to agricultural land chiefly because of its fertilizing value. Animal manure supplies all major nutrients (N, P, K, Ca, Mg, S,) necessary for plant growth, as well as micronutrients (trace elements), hence it acts as a mixed fertilizer (see Annex IV) The fertilizing effect on crops can be compared to the effect of mineral fertilizers, and expressed in working coefficients. If, for example, the N in pig slurry on maize is half as effective in terms of yield increase as the N from ammonium nitrate (which is the reference chemical fertilizer), the working coefficient is 0.5. Manure application in a given year will influence not only crops grown that year, but also crops in subsequent years, because decomposition of the organic matter is not completed within one year. Working coefficients for subsequent years could be determined as well. Therefore, the application of manure, thus, saves mineral fertilizers for various nutrients. This illustrates that nutrients from animal manure can be substituted for mineral fertilizers and which is far better for the environment.
A disadvantageous aspect of the uptake of components from manure by the crop is over-dosage, which can lead to the absorption by plants of non-degradable components such as heavy metals (Cu, Zn) and organo-chlorines. These components can accumulate in the food chain and become a health hazard.
2.7.3 Hydrological conditions
The effects of manure on the environment are strongly influenced by the prevailing climatic, soil and hydrological conditions, which are dealt with in this section. In Chapter 3 rough estimates are made of the extent to which these conditions influence processes such as emission, leaching and crop utilization of nutrients from manure.
The most important climatic characteristics affecting processes such as emission, leaching and decomposition of organic material are temperature, precipitation and evapotranspiration. Temperature strongly influences all microbiological processes. Higher temperatures, therefore, lead to higher rates of nitrification, denitrification and decomposition of organic material, but also to faster crop growth and the associated uptake of nutrients from manure. Nitrates are formed faster and are therefore more susceptible to leaching, but are also taken up faster by plants if a crop or vegetation is present. At higher temperatures and under reduced conditions NO3 will, be denitrified more rapidly and N2O, (harmful to the ozone layer), will be formed more quickly.
The extent of nutrients leaching to the ground water is largely determined by the balance of precipitation and evapotranspiration. If, for a certain period precipitation exceeds evapotranspiration considerably, NO3, P, other nutrients and organo-chlorines can leach to the ground water. Conversely, if annual potential evapotranspiration greatly exceeds annual precipitation, upward movement of water in the soil will occur, with the imminent danger of salinisation. High doses of manure could add such large amounts of nutrients that they cannot all be taken up by the crop. Nutrients then accumulate and cause salinisation (Sequi and Voorburg, 1993). Potential evapotranspiration (PET) also strongly influences the rate of NH3 volatilization; high PET leading to high rates of volatilization.
Of the soil characteristics, only texture and pH will be discussed. The water and nutrient holding capacity of clay soils is higher than that of sandy and silty soils, therefore leaching of NO3, P, other nutrients and organo-chlorines is dependent on soil texture. Conversely the risk of accumulation of harmful components in the root zone following repeated application of large doses of manure is higher in heavier textured soils than in light soils. Clay soils become more easily waterlogged after heavy rainfall because of a lower hydraulic conductivity, i.e. the possible rate of water transport through the soil. Under waterlogged conditions, denitrification can occur and harmful N2O may be formed. Under extreme acid or alkaline conditions (pH<4 or>9), soils tend to deflocculate, the structure is destroyed and leaching of many organic and inorganic components becomes inevitable. Volatilization of NH3 from soils with higher pH values is greater than from those with lower pH values.
The hydrological conditions strongly influence the leaching process, for example, faults or sinks in karst zones can cause leaching into deep, ground water-carrying layers. Pollutant containing topsoil can come into contact with deep aquifers via old ground water wells.
The flow from ground water to surface water is usually direct in areas with a shallow ground water level. In this way, leached nutrients/pollutants can flow rapidly from the soil via the ground water to the surface water and cause eutrophication and toxicity problems.